The proposed study will evaluate the coronary vasodilator activity of exogenous and endogenous prostacyclin. In open-chest anesthetized dogs, and in closed-chest sedated dogs, coronary blood flow responses to intracoronary and intravenous prostacyclin, prostaglandin E2, nitroglycerin, nitroprusside, dipyridamole, norepinephrine, and adenosine will be measured by flowmeter and by radioactive microspheres in normal hearts and in hearts with graded levels of coronary occlusion. Some dogs will be pretreated with aspirin. Prostacyclin levels in coronary sinus blood samples will be determined by radioimmunoassay for 6-ketoprostaglandin F1. A profile of various types of vasodilator activity in non-occluded and in partially occluded vessels will be established. Extent of prostacyclin mediation of the vasodilator activities of nitroglycerin, nitroprusside and dipyridamole will be estimated. Potentially deleterious or beneficial vasodilator-aspirin interactions will be identified. These studies will also evaluate modulation of coronary vascular tone and responsiveness by endogenous prostaglandins. In a separate series of studies, coronary vasodilator effects (flow probe and microsphere determinations) of combinations including norepinephrine plus prostaglandins, tyramine plus prostaglandins, tyramine plus aspirin, selective adrenergic antagonists plus prostaglandins will be examined. Prostaglandins will include prostacyclin and prostaglandins of the E, F, and A series. The drugs will be administered directly into non-occluded and partially occluded coronary arteries. Interactions will be studied in untreated dogs, in dogs pretreated with aspirin, and in sympathectomized dogs. The series of studies should identify coronary vascular interactions of the prostaglandin system with the sympathetic nervous system.